This invention relates to rotary cutting tools and more particularly to helically fluted end mills and ball nose end mills with diamond-like cutting edges.
Helically fluted end and ball nose end mills are commonly used milling tools and are generally required to perform severe machining operations under adverse conditions as well as finishing operations where a fine surface is desired. The cutting end of a helically fluted end mill, for example, includes at least one cutting edge on the end mill blank. The cutting end of a ball nose end mill carries the cutting edge around the hemispherical end of the mill.
Oppositely directed cutting surfaces positioned at the cutting end of the mill blank are subjected to axial and torsional loads which create demands on the materials used for fabrication of the milling tool. Clearly the material of the cutting edge should be as hard as possible to cut a workpiece and it should also be heat resistant to maintain the cutting edge of the mill at elevated temperatures. Moreover, the material of the body of the mill blank must be both rigid and tough to resist deflection and to maintain the integrity of the mill under loads while the end mill is being used. The foregoing requirements have resulted in compromises in material selection since hard materials tend to be brittle while tough materials tend to wear quite easily.
This invention has application for other types of rotary cutting tools such as router bits, reamers and taps which may have cutting surfaces on an end face.
The prior art teaches a combination of materials having the characteristics of hardness and wear-resistance at the cutting surfaces and toughness and rigidity of the body and shaft. It has been previously proposed to form the cutting surfaces of one material and the body and shaft of another. This has resulted in a variety of combinations such as cemented tungsten carbide or diamond inserts or tips on carbon steel or carbide shafts. These combinations while individually useful have a common disadvantage, i.e. the braze connection between the insert or tip and a shaft.
Tungsten carbide can be soldered or brazed directly to the steel or shaft. However, diamond must first be adhered to a carbide substrate which is in turn soldered or brazed to the shaft. Diamond particles are typically formed into a compact or PCD (polycrystalline diamond) disk and bonded to a carbide substrate with a metallic catalyst in a high pressure-high temperature press. At atmospheric pressures, however, the metal which catalyzes the bonding of the diamond particles to each other and to the substrate in the press will also catalyze the conversion of diamond to graphite at temperatures above 700.degree. C. which will cause disintegration of the PCD compact. Accordingly a low temperature solder or braze connection is used to attach the substrate to the shaft. The aforementioned diamond disks as well as the diamond insert stud blanks, for example, are fabricated from a tungsten carbide substrate with a diamond layer sintered to a face of a substrate, the diamond layer being composed of polycrystalline material.
A suitable synthetic polycrystalline diamond layer is manufactured by Megadiamond Industries, Inc., Provo, Utah.
Two examples of patents assigned to Megadiamond describe cutting elements for drilling holes. U.S. Pat. No. 4,527,643 describes a cutting element for drilling holes which consists of five cutting edges which are comprised of polycrystalline diamond or the like mounted to a central carbide substrate of similar hard material held by a rotatable shaft which can be inserted into a drilling machine. The polycrystalline material is then supported with respect to torsional forces exerted upon it during drilling.
U.S. Pat. No. 4,627,503 describes a polycrystalline diamond and metal element for use as a cutting element for drilling holes or similar uses. The cutting element comprises a polycrystalline diamond center portion sandwiched between metal. The metal side portion is made from a soft metal having a Young's Modulus less than approximately 45.times.10.sup.6 and is selected from a group comprising cobalt, nickel, iron, copper, silver, gold, platinum, palladium and alloys of these metals.
Both of these patents utilize a braze type bonding element to secure the diamond cutters within a drill blank. Typically a low temperature solder or braze connection is used to attach the substrate to a shaft such as the shaft of a helical twist drill. This braze connection limits the effective life of such drilling tools since it is softer than either the substrate or the shaft. Thus, the braze becomes the weakest point of the tool construction and the limiting factor in the tool usage.
U.S. Pat. No. 4,762,445 teaches a helically fluted twist drill apparatus in which offset opposed veins of sintered abrasive particulate, such as diamond, are embedded within a drill blank made of a less abrasive material such as carbide. The non-aligned veins of abrasive material, themselves intersect adjacent the point and web of the drill. The veins of diamond are 180.degree. opposed across the tip of the helical drill blank. The opposing veins intersect at the center or axis of the helical drill to provide a concentration of diamond at the tip of the twist drill.
U.S. Pat. No. 4,991,467 describes a diamond tipped twist drill for drilling holes in a workpiece. A drill blank body has a pair of flutes, each flute including a channel that essentially parallels the flutes. Each channel ends at an aperture formed in the body nearest a cutting end of the drill. Diamond material is pressed into the grooves and through the aperture. Subsequent machining at the cutting end of the drill bit body exposes the diamond at the cutting tip and the diamond adjacent the leading edge of the flutes.
U.S. Pat. Nos. 5,031,484 and 5,070,748 describe an end mill having at least a pair of spiral grooves or flutes in the mill blank side walls. Each groove includes polycrystalline diamond or polycrystalline cubic boron nitride formed along a leading edge of each flute. The end mills described by the foregoing patents do not have center cutting capabilities.
U.S. Pat. Nos. 4,991,467; 5,031,494 and 5,070,748 are hereby incorporated by reference.
The present invention overcomes the problems of the foregoing prior art by providing, for example, a concentration of diamond on a flute and across the end face of a milling cutter blank.